1. Field of the Invention
The invention relates to a method of performing forming operations on steels, metals, and alloys having low deformability and/or high resistance to deformation at room temperature, wherein the workpiece stock materials are particularly hardenable steels, e.g. high speed tool steels, the thickness of the stock material is small, preferably less than 10 mm, and the overall reduction of the cross section in the process is substantial. The invention further relates to an apparatus comprised of a heating device, a temperature equalization and guiding device, and a forming apparatus for carrying out the method.
2. Description of the Prior Art
Wire, rod, tube, and profiles, of small diameter, and possibly with thin walls, are customarily manufactured by a staged process comprising, first, hot forming of the stock, second possibly soft annealing, and then cold rolling or cold drawing. In most cases the thickness of the stock material is less than 10 mm.
In the course of cold forming, the material is hardened. As the degree of hardness increases, the ductility decreases and the resistance to deformation decreases. The limit of deformability is reached at low degrees of overall deformation. Materials which have high ductility at room temperature, and thus high cold deformability, can undergo high degrees of deformation in cold rolling and cold drawing, with decreases in cross section of, e.g., 10:1 (i.e. 90%) or more. In a case where the material has low cold-deformability and therefore hardens during the course of deformation such that it becomes impossible to process it further, i.e. by further cold rolling or cold drawing, to reach the desired final dimensions, and cracking and breakage occur due to exceeding the limits of deformability, in order to continue, the hardening must be reversed by heating to appreciable temperatures, or a stage of annealing must be resorted to. Such intermediate heat treatment breaks down the hardened structures in the material. For hardenable steels, particularly air-hardening steels such as tool steels and high speed tool steels, the intermediate heat treatment may comprise soft annealing. For economic reasons, however, in most cases any annealing must be for an extended period, possibly at a temperature below the austenitizing temperature or below the AC-1 point of the alloy, AC-1 being defined for purposes of this description as meaning the temperature at which austenite begins to be formed upon heating a steel.
In general if one is employing stock material having low deformability, such material cannot be subjected to forming operations to the desired final dimensions while in a ductile state at forging temperatures, because there is radiation energy loss from the surface, which radiation increases with the 4th power of temperature, and this energy loss leads to low temperatures in the zones near the surface, resulting in grain transformations and hardening of the material (in the case of hardenable steels and alloys). The reason for the rapid cooling which occurs is the low heat content of the material, in consequence of the small cross sectional area. Further, after a forming operation is performed on, e.g., hardenable alloys at forging temperatures, soft annealing must be carried out.
In order to avoid the need for heat treatment after forming, and to avoid hardening (where hardening is a limiting problem), and in order to increase the deformability and thereby to increase the degree to which the cross section of the material can be reduced, it has been proposed to carry out the forming at elevated temperature, subject to a possible upper limit of the austenitizing temperature or the AC-1 point of the alloy. A difficulty faced in this proposal is that of ensuring that the deformation energy applied in a given cross sectional region does not itself lead to a temperature increase to a point above the AC-1 point.
Drawing has proved to be advantageous for the deformation process at elevated temperature, because energy is produced by the friction in the drawing die and by the principal deformation in the zone of the stock near the surface thereof, and this energy substantially completely compensates for the radiation losses. The temperature distribution over the cross section is improved, i.e. is more uniform which enables greater degrees of area reduction to be achieved per forming step. If a plurality of drawing steps is employed, to enable achieving a high overall degree of reduction of the starting cross sectional area, and if one achieves this increase in the reduction of cross sectional area per step, then the number of drawing passes can be reduced, along with the number of de-hardening steps, which steps are carried out between respectively successive drawing passes. However, the drawing speed of the materials at elevated temperature must be kept low, e.g. 0.2-2 m/sec, because otherwise excessive wear on the drawing die is experienced due to forcing away of the lubricant film; and further, the time required for the preparatory heating and de-hardening of the material is long, necessitating uneconomically long heating segments in the system.
As an example, a soft-annealed high speed tool steel wire (material DIN No. 1.3343) with a diameter of 5.5 mm can be continuously drawn from a reel into a lead bath with a length of 10 m and a bath temperature of 700.degree. C., with a residence time of 20 sec in the bath, to heat and anneal the material. This is followed by drawing in a drawing die to a diameter of 4.7 mm, with a speed of 0.5 m/sec, following which the wire is re-spooled. The deformation experienced is about 27%. The wire is thereafter brought to a diameter of 1.6 mm, in seven further similar drawing steps, and four de-hardening annealing steps may be included between pairs of the seven drawing steps, these intermediate annealings being carried out under oxidation protection at 800.degree. C. and an annealing time of 1 hr. each. For each drawing step, a lead bath is employed prior to the drawing, to bring the material to temperature and further relax any hardening.
It is a disadvantage to have to employ a large number of steps with intermediate de-hardening annealing, when forming by drawing at elevated temperature, when the stock comprises steels or alloys, particularly hardenable steels, of relatively thin dimension. The arrangement is costly, the drawing speeds are low, there are problems with high temperature drawing means and agents (lubricants etc.), and the wear on the dies is high.